
178 Power System Transients
(PW) reaches its electrical withstand voltage, a ashover from the tower
to the PW occurs. This is called “back ashover” (BFO), because the tower
voltage is higher than the PW voltage, unlike normal power system opera-
tion. Then, the lightning current ows into the PW and a traveling wave
due to the current propagates to a substation. The traveling wave is partially
reected at and refracted into the substation. The traveling wave or its suc-
cessive reection generates a severe overvoltage at the substation.
2.4.2 Modeling of Circuit Elements
2.4.2.1 Lightning Current
Lightning is often modeled by a current source ...